This question will almost certainly be cleared up in the Anime; the Gomu Gomu no Storm is hitting all the Marine at once as they converge on him. This knocks falling Marines into other Marines, and hence, how quickly they are being defeated. Even if inaccurate (I cannot confirm this; wasn't this the attack Luffy beat Crocodile with, hence, pretty accurate), there are so many Marines he hits a couple with each punch.
Also, since Luffy's attacks revolve around stretching, I do not see why people are surprised if a crowd killing attack is upgraded and called something different. Although Luffy does use new moves, think about his power. -.-;
Luffy's already showed many more original moves than any other single Devil Fruit user, and he came up with a new one; that Gomu Gomu no Robo attack.
Personally, I'm hoping he attatches to a guy with a minigun and uses that technique for a three hundred and sixty degree spray. If Luffy winds up around the guy and uses the Robo technique…ahh, does anyone else have an image of the Enies Lobby rivers running red? =D